This description relates to managing leakage inductance in a power supply.
A variety of types of power supplies (also called power converters) provide an approximately constant (i.e., DC or “direct current”) output voltage from a sinusoidal (i.e., AC or “alternating current”) input voltage (e.g., from an AC mains source), thus providing AC-DC conversion (also called “rectification”). Other types of power supplies provide AC-AC conversion, DC-DC conversion, or DC-AC conversion (also called “inversion”). Some power supplies include multiple stages, which may include a combination of different types of converters, such as a DC-DC converter stage within a DC-AC power supply. Converter stages that provide a DC voltage may use a regulator to maintain the DC voltage near a constant value. A linear power supply uses a linear regulator, and a switched-mode (or “switching”) power supply uses a switching regulator that switches between on and off states to regulate power transfer and maintain the DC voltage.
Switched-mode power supplies can be classified as having different types of circuit topology. For example, some topologies include one or more stages that have certain arrangements of components that perform specific functions (e.g., buck, boost, or buck-boost stages). Some “isolated” circuit topologies provide “isolation” using a transformer to prevent certain current flows between isolated stages of the power supply. Examples of different circuit topologies that can be configured to provide isolation include: full bridge, half bridge, and push-pull. These circuit topologies correspond to different arrangements of switches, and can be used in either isolated or non-isolated power supplies. Another example of a circuit topology that can be configured to provide isolation is flyback, which refers to a specific way of using a transformer. Certain classes of power supply are also identified by certain specific features of their circuit topology. For example, a power supply (switched-mode or linear) that operates directly from an AC mains source is called an “off-line” power supply.
Some power supply circuits use power factor correction (PFC) to increase the circuit's “power factor” which can be defined as the ratio of the total “active power” (the true power being drawn from the source) to the total “apparent power” drawn from the source (based on a vector sum of the true power and the reactive power at the output). Increasing the power factor reduces losses incurred in the upstream power distribution system. Often, power supplies with PFC functionality include two stages: a front end boost converter controlled as a PFC stage, and an isolated DC/DC converter as a second stage. Some two-stage converters have a number of positive attributes, including reduced total switch root mean square (RMS) current for a given output power, but tend to entail significant complexity, with many semiconductor switches and passive components (e.g., inductors and capacitors). For some applications, the ability to supply large peak power is useful. Power supplies for audio applications, for example, typically need to supply peak powers of at least 3 times the average power—other attributes besides RMS currents at maximum output can dominate the economics and desirability of a power supply under these circumstances.
Transformers used to isolate stages in these power supplies exhibit what is called “leakage inductance”, representing an imperfect coupling of magnetic field between the primary and secondary windings of the transformer. Some amount of leakage inductance is inevitable—it is impossible to have two coils experience perfect magnetic coupling. The effect of leakage inductance on the push-pull topology, for example, is generally to add voltage spiking on the drains of the switches, requiring extra voltage margin—this is one of the reasons that this topology is rarely used for off-line power supplies. Circuitry such as a dissipative or non-dissipative snubber can be used to alleviate this problem, but there are limitations to that approach.